Tropical Storm Arthur has strengthened steadily on Tuesday, with maximum sustained winds of 70 miles per hour, just below hurricane strength — which is 74 miles per hour. The National Hurricane Center has upgraded hurricane watches to hurricane warnings for the North Carolina coast, which means that hurricane conditions are expected within 36 hours.

The latest computer models have shifted the storm track slightly westward, increasing the chances that the storm may briefly make landfall along the Outer Banks sometime between Thursday evening into Friday morning. The issuance of hurricane warnings may prompt local emergency management officials in eastern North Carolina to order evacuations of vulnerable low-lying locations and barrier islands that can flood easily during hurricanes. Emergency officials ordered on Wednesday night a mandatory evacuation of Hatteras Island, located along North Carolina's Outer Banks, as Tropical Storm Arthur approaches. The evacuation is slated to begin at 5 a.m., local time, Thursday, after which no one will be allowed on the fragile barrier island.

One thing weighing on officials' minds is that storm intensity forecasts tend to be less accurate compared to track forecasts. It is possible that the storm could be a Category Two storm on the Saffir-Simpson Scale if conditions are ideal for rapid intensification, which would entail a more damaging storm surge potential.

For much of the East Coast, it's going to be a wet and breezy Fourth of July, thanks to the first tropical storm of the season — plus a cold front pushing in from the Midwest. This means many fireworks displays will be postponed (sorry, kids).

As of 11 a.m. ET on Wednesday, Tropical Storm Arthur was spinning northward off the east coast of Florida, with nearly all computer models projecting a turn to the northeast and strengthening to a minimal hurricane within the next 36 hours. While storm intensity forecasts are far less reliable than track projections, the storm has been forecast to become a Category One hurricane since it formed on Tuesday.

Visible satellite image of Tropical Storm Arthur on the afternoon of July 2, 2014.

Image: NOAA Satellites

Currently, the storm contains maximum sustained winds of 60 miles per hour. A hurricane has winds equal to or greater than 74 miles per hour.

The storm threatens many family vacations during the Fourth of July holiday, particularly for anyone hoping for a beach day on Friday in the Carolinas. The National Weather Service has issued tropical storm warnings and hurricane watches for parts of eastern North and South Carolina, and is projecting a relatively small storm surge threat of about two to four feet of water above ground, depending on whether the storm hits at high tide. (To monitor the storm surge forecasts, check out the Hurricane Center's new experimental surge forecasts, which were developed in the wake of Hurricane Sandy in 2012.)

Tropical storm and hurricane conditions may affect the areas under watches and warnings as soon as Thursday night.

The National Hurricane Center in Miami warned that "any deviation of the forecast track to the left, or an increase in the forecast size of Arthur would likely require the issuance of hurricane warnings for all or part of the hurricane watch area."

Computer model track forecasts for Tropical Storm Arthur, made on July 2, 2014.

The worst-case scenario involves a brief landfall in eastern North Carolina on Thursday night or Friday morning, followed by a close pass to southeastern New England.

As of Wednesday morning, Arthur was still entraining some dry air into the western side of its circulation, leading to a lopsided appearance with the largest thunderstorms and strongest winds confined to the eastern edge of the circulation. If this continues, it may not intensify as much as currently forecast.

The storm, be it a hurricane, tropical storm, or a hybrid, is not projected to make landfall anywhere in the Mid-Atlantic or New England, although people in this region need to keep a close eye in case the forecast changes. This is particularly true for residents and vacationers on Cape Cod and the islands of Nantucket and Martha's Vineyard, which are within the Hurricane Center's "Cone of Uncertainty" forecast map. The highest odds of seeing tropical storm or even hurricane-force winds will be confined to the immediate coast of North Carolina, and potentially the Cape and Islands of Massachusetts.

Ocean temperatures off the Mid-Atlantic and New England are well above average for this time of year, and could support a hurricane even though it is just the beginning of the hurricane season. This region typically sees such storms later in the season, after the ocean has had a chance to warm up after the winter.

Sea surface temperature anomalies off the East Coast, showing milder than average water off the Mid-Atlantic and New England.

Image: NOAA

If the storm is not likely to make a direct hit anywhere other than, perhaps, North Carolina's Outer Banks, why is it still likely to spoil many people's Independence Day weekend plans?

The East Coast won't just be dealing with Arthur on Thursday and Friday, but rather a rain-producing duo of the tropical weather system and a slow-moving cold front that has helped spark massive thunderstorms in the Midwest over the past week.

According to the National Weather Service, thunderstorms from this frontal system will interact with some of the moisture streaming northward from Arthur to dump several inches of rain along the I-95 corridor during Thursday and Friday. The details of how the front will interact with Arthur are difficult to forecast ahead of time, except to say that it's likely it will enhance rainfall from Washington to Boston.

The National Weather Service said that the front is likely to bring the heaviest rain to the eastern seaboard, with lower amounts inland. Some of the moisture from Arthur could stream northward, adding to the already moisture-rich environment ahead of the front. In an online forecast discussion, the NWS said:

Showers and thunderstorms will continue to ignite along a frontal boundary gradually pressing eastward out of the Ohio Valley and into the Northeast and Mid-Atlantic states. A warm and unstable airmass in place ahead of the front could allow storms to become organized and potentially capable of producing severe weather. Also...as the boundary edges far enough eastward and begins interacting with moisture from Tropical Storm Arthur...heavy rains with localized flash flooding will become a possibility.

The good news, though, is that both the front and Arthur will clear the East Coast by Saturday, leaving a gorgeous rest of the holiday weekend.

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